The experience of recent decades has challenged the prediction that the single currency would help differences in income levels across euro area countries narrow over time. This income convergence among the founding countries of the euro has not happened, prompting a need for further economic reforms. While newer members of the euro have converged, even this trend has stalled since the crisis.

As the crisis in Europe deepens, it is worth asking how it all went wrong in the first place. In the past decade there have been stark differences in per capita GDP growth in Europe. Growth rates have ranged from close to zero in Italy and Portugal to more than 4 percent in the best performers. Why do some countries in Europe grow much faster than others? And how can those falling behind catch up before it is too late?

In part, these differences reflect “convergence”. It is much easier for poor countries to grow faster than it is for rich countries because they can import technology they do not already have. It is much more difficult to grow fast if you are already rich and at the technology frontier—now you can only get richer by innovation.